The invention relates to means for connecting a front panel of a parallelepiped built-in unit to a built-in housing, the front panel being assigned to a narrow side of the built-in housing.
If the abovementioned built-in unit is a tachograph used in a motor vehicle, then the front panel thereof, in addition to satisfying esthetic requirements, also constitutes a functional part. To better serve the functionality, a window cut-out for a display, button shafts and guide slots assigned to the data cards of the driver and passenger, and possibly also a discharge slot for a printout are provided.
The connecting means between the front panel and the built-in housing of such a tachograph have to have a high level of positioning accuracy in relation to the subassemblies arranged in the built-in housing, i.e. the display, various buttons, a printing unit and the data-card assemblies. Additionally, the connecting means have to avoid changes in position as a result of material fatigue on account of vibrations which are inevitable during operation of a motor vehicle and on account of the fluctuations in temperature which prevail at the installation location. On the other hand, the recordings of tachographs, in particular the recordings relating to the working and rest periods, are at risk of manipulation, with the result that it has to be ensured that tampering which cannot be subsequently detected, for example through a gap between the front panel and built-in housing, is reliably prevented.
Quite apart from the fact that they are not suitable for mass production on account of the outlay in respect of components and installation, it is also the case that conventional screw connections secured by lead seals do not fulfill these conditions, inter alia, because the amount of installation space which is necessary for fitting a plurality of screw connections on the front side is not available and, as experience has shown, the lead-seal caps which secure the screws do not perform a reliable sealing function. In contrast, hooks or claws integrally formed on the front panel, in order to be sufficiently resilient for a latching connection between the front panel and the built-in housing, require a certain length and amount of free space. In addition to the risk of such connecting means breaking, this type of fastening is tolerance-critical, with the result that a front panel, in some circumstances, cannot be installed or an unstable connection which is no longer free of play is possibly established over time on account of vibrations and extreme change in temperature.